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The breakthrough represents a major accomplishment in the $390.8 million project that aims to relieve congestion on state Highway 24 between Orinda and Oakland.

Project officials said Monday that crews were just a few meters shy of breaking through and that the work was dependent on ground conditions within the tunnel.

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Morrison said the excavating machine punched through the last barrier at about 2:45 p.m. Tuesday.

Crews have been excavating the top portion -- or the topheading -- of the fourth bore, although much more work lies ahead before the project is scheduled to be completed in late 2013, Morrison said.

The project is a partnership between the Federal Highway Administration, Caltrans, the Metropolitan Transportation Commission, the Contra Costa Transportation Authority, and the Alameda County Transportation Commission.